1. Field of the Invention
The present invention relates to a resistance change memory, e.g., a write-system circuit of a resistance change memory.
2. Description of the Related Art
A magnetoresistive element is known as a nonvolatile memory element. The magnetoresistive element includes a free layer and fixed layer as magnetic materials, and a nonmagnetic layer sandwiched between them, and takes different resistance states in accordance with the directions of magnetization in the free layer. A magnetoresistive random access memory (MRAM) is a resistance change memory that stores data by using the difference between the resistance states.
As a data write method, a so-called spin transfer torque writing method different from the conventional so-called magnetic field writing method using a current magnetic field has been proposed (e.g., U.S. Pat. No. 5,695,864).
In spin transfer torque writing, a flow of electrons spin-polarized by the magnetic moment in the fixed layer of the magnetoresistive element is supplied to the free layer. Since the magnetization direction in the free layer changes in accordance with the direction of this electron flow, specific data is written in the magnetoresistive element. Unlike the magnetic field writing method, the spin transfer torque writing method can directly act on the magnetoresistive element. Therefore, no write error occurs in any adjacent memory cell. There is another advantage that the current amount necessary for write reduces as the cell size decreases.
Data is read out by supplying a read current to the magnetoresistive element, converting the resistance value into a current value or voltage value, and comparing the value with a reference value.
The magnetic field writing method and spin transfer torque writing method use the same mechanism to hold data in the magnetoresistive element. Accordingly, a partial peripheral circuit configuration can be used in both MRAMs using the two writing methods. However, since the principles of write are different, the two writing methods require some different peripheral circuits. Therefore, demands have arisen for the implementation of a write-system circuit capable of generating a high-accuracy write voltage suited to the spin transfer torque writing method.